Over the past 30 years, there have only been a handful of elections where a governor made a difference in a caucus or primary outcome.

As the first presidential caucuses and primaries of the 2016 cycle creep closer, we wondered: How often have governors been able to boost a presidential candidate to victory in their state?

Political strategists have long argued that it's a good idea for presidential candidates to court as many governors for endorsements as they can.

"If governors of a particular party across the country endorse one candidate, then that is a signal of the institutional support within the party establishment," said Joshua T. Putnam, a University of Georgia political scientist.

But history suggests that rhetorical support from a governor doesn't necessarily translate into primary or caucus success.

Based on interviews with a wide variety of political observers in key primary and caucus states, we found only three, possibly four, examples in the past 30 years in which a governor helped a presidential candidate win the nomination. (We found one additional, older example that we aren't including: California Gov. Earl Warren's support to Dwight Eisenhower in 1952.)

One reason governors might not have as much influence on primary or caucus victories is that many refrain from picking sides in a contested primary, said Timothy M. Hagle, a University of Iowa political scientist.

"Governors will often choose to not get involved in primaries until it's pretty clear who the winner will be," Hagle said. "It's rare in part because the governors won't want to seem weak by picking a loser."

This standoffishness is especially common in states such as Iowa, where the importance of the first-in-the-nation caucus depends on encouraging as many candidates to come and campaign, rather than seeking to winnow the field prematurely.

"In Iowa, our governors have generally not endorsed presidential candidates in the lead-up to the caucuses because they've wanted to make sure that all the candidates feel welcome here," Hagle said.

Even when governors do make a presidential endorsement in a heated primary contest, they don't necessarily bring assets on the ground to bear, said Wayne P. Steger, a DePaul University political scientist.

"There may be some influence here and there, but it really depends on how much time and energy a governor expends on behalf of a presidential candidate lining up donors and support within the state," Steger said. "Most governors' endorsements are largely symbolic statements of support without necessarily the concomitant campaign support."

Indeed, there's slender evidence that governors can make a difference in results. In a 2002 paper that looked at general elections rather than primaries, Emory University political scientist Alan I. Abramowitz found little correlation between gubernatorial party control in a state and the ability of the candidate from that governor's party to win that state.

"Despite the strong approval ratings and high re-election rates enjoyed by the nation's governors during the late 1990s, there is no evidence that they were able to transfer their popularity to their party's presidential candidates," Abramowitz's calculations showed.

So what are the races where a governor's support made a difference? We detail them below. Did we miss any? If so, drop a line to ljacobson@tampabay.com.

The 1988 New Hampshire Republican Primary

"The single best example in New Hampshire -- maybe the only example in New Hampshire -- was Republican Gov. John H. Sununu's support and work on behalf of George H. W. Bush in 1988," said Jamie Burnett, a New Hampshire-based GOP strategist.

Sununu, in a 2007 op-ed, recalled that then-Sen. Robert Dole of Kansas was stepping up attacks on his rival, Bush, "charging that the vice president didn't believe in cutting taxes and wasn't a worthy successor to Ronald Reagan." In response, Bush's consultants, Lee Atwater and Roger Ailes, had prepared an ad -- now known as 'Senator Straddle' -- attacking Dole for having voted for tax increases.

The campaign debated whether to use it. Sununu and others urged Bush to run it. Once the decision was made, Sununu used his connections to prevail upon the state's lone television station WMUR to accommodate the ad placement.

"Sununu's endorsement and on-the-ground efforts made an enormous impact on Bush's race and turned it around from an underwhelming campaign to a successful reboot that took him all the way to the White House," Burnett said. "This is by far the biggest impact that any state politician ever had on a presidential campaign in the Granite State."

The 1992 Georgia Democratic Primary

In 1992, Georgia's Democratic Gov. Zell Miller pushed the legislature to move up the date of the 1992 presidential primary to March 3, one week before that year's Super Tuesday primaries. The idea was to give Bill Clinton -- a fellow Southerner -- a leg up on Super Tuesday, when other Southern states would be voting, including Florida, Louisiana and Mississippi.

Clinton -- who had not finished in first place in any of the previous primaries or caucuses -- won 57 percent of the vote in Georgia, and was able to parlay that into wins in every Super Tuesday state except for Massachusetts, the home of one of his rivals, the late Sen. Paul Tsongas.

"Clinton would have fared pretty well in the Southern states anyway, but Georgia was definitely a trigger," said longtime Georgia political journalist Tom Baxter.

The 2008 Florida Republican Primary

In 2008, Florida Republican Gov. Charlie Crist endorsed Arizona Sen. John McCain in the waning days before the Florida primary.

Crist "had been flirting with endorsing Rudy Giuliani," who had essentially staked his candidacy on Florida, said Jim Barnes, co-author of The Almanac of American Politics 2016. But at the last moment, Crist endorsed McCain instead. The Arizona senator proceeded to notch a narrow victory over former Massachusetts Gov. Mitt Romney and went on to win the nomination.

The 2004 Iowa Democratic Caucus

This one deserves an asterisk because the endorsement in question didn't come from Democratic Iowa Gov. Tom Vilsack, but rather his spouse, Christie Vilsack.

That year, the governor declined to make an official caucus endorsement. But Christie Vilsack, in what was seen by many observers as an unofficial show of support by governor himself, endorsed John Kerry shortly before the caucuses were held, even though the state's Democratic senator at the time, Tom Harkin, had already endorsed then-frontrunner Howard Dean. Bolstered by Christie Vilsack's endorsement, Kerry went on to a somewhat unexpected win in the caucuses, enabling him to muscle Dean aside (on the night of the famed "Dean scream") and get the Massachusetts senator started on his march to the nomination.